1. Field of the Invention
The present invention relates to an apparatus for charging a rechargeable battery. More specifically, the present invention relates to an improvement in an apparatus for charging a gas tight sealed rechageable battery which is adapted to be responsive to a charged state of the battery to stop charging the battery.
2. Description of the Prior Art
A typical prior art apparatus for charging a gas tight sealed rechargeable battery which is adapted to be responsive to a charged state of the battery to stop automatically the charging of the battery comprises a potential divider connected in parallel with the battery for the purpose of detecting the terminal voltage of the battery and a threshold detector for detecting a predetermined threshold level selected to be representative of a charged state of the battery, and a control circuit responsive to the threshold detected output for terminating a charging current or changing a charging current from a rapid charge mode to a trickle charge mode.
A problem encountered in such a prior art battery charging apparatus is that the above described threshold level to be selected to be representative of a charged state of the battery is not necessarily associated with the state in which the battery has been fully charged. This will be described in more detail with reference to FIG. 2, which shows a charge voltage characteristic (A) and a charge quantity characteristic (B) and also shows a voltage characteristic (C) of a charged state detecting voltage developed in accordance with the present invention, as to be more fully described subsequently. It is pointed out that FIG. 2 shows such characteristics obtained with an alkalin battery, such as a nickel-cadmium battery. Referring to FIG. 2 and particularly to the characteristic curves A and B, a time point t1 when a charge quantity characteristic B becomes saturated comes after a time point t2 when a charge voltage characteristic A passes a peak point a on the characteristic curve A. In view of the fact that the charge voltage characteristic curve A shows a rise slope before the peak point a whereas the curve A shows a fall slope after the peak point a, a typical prior art automatic battery charging apparatus has been adapted to detect a predetermined threshold voltage level V1 which is selected at a rise slope portion of a larger gradient of the characteristic curve A. As readily seen from the FIG. 2 illustration, a time point t3 when such a predetermined threshold voltage level V1 is detected comes before the time point t2 when the characterisitic curve A reaches the peak point a. As readily understood from comparison of both curves A and B with the timing relation of the time points t1, t2 and t3, such a prior art automatic battery charging apparatus is disadvantageous in that a charged state of the battery is detected when the charge quantity characteristic has reached a value much lower than the saturated value of the charge quantity.
An approach has also been proposed in which a timer is adapted to be responsive to the threshold detected output obtained at the time point t3 corresponding to the predetermined threshold voltage level V1 which timer is structured to determine a time period t3-t2 so that a timer output is obtained at the time point t2 corresponding to the peak of the charge voltage characteristic where threshold voltage level detection is extremely difficult, thereby to stop the charging operation at a state in which the charge quantity characteristic has reached a larger value or the battery has been much more charged. However, this approach requires a timing mechanism or a timing circuit which makes the apparatus expensive in cost. Another disadvantage in this approach is that in view of diversified characteristics of the batteries per se consideration should be given so as to stop charging the battery at a lower average charge quantity value in order to avoid overcharging in some batteries which are likely to be charged more rapidly than others. Hence, as an average, it is difficult to charge the batteries of such diversified characteristics such that each battery is charged individually as fully as possible. Another reason why a time period achieved by a timer cannot be selected to be longer is that if a battery of a relatively large remaining quantity of electricity is charged for such a time period determined by a timer it could happen that the battery is overly charged.
Another problem encountered with a conventional automatic battery charging apparatus is that since a non-active battery is likely to show a higher charge voltage characteristic at an earlier stage in the charging operation of the battery such a type of battery is difficult to be charged with a conventional apparatus. A non-active battery may be defined as a battery which is apparently difficult tentatively to be charged, a battery which is apparently lower tentatively in charge efficiency, a battery which does not tentatively give rise to a chemical reaction, and so on.